This invention relates in general to cold cathode gas discharge devices, and in particular, to a high power cold cathode gas discharge system.
Hot cathode fluorescent lamps (HCFLs) have been used for illumination. While HCFLs are able to deliver high power, the useful life of HCFLs is typically in the range of several thousand hours. For many applications, it may be costly or inconvenient to replace HCFLs when they become defective after use. It is therefore desirable to provide illumination instruments with a longer useful life. The cold cathode fluorescent lamp (CCFL) is such a device with a useful life in the range of about 20,000 to 50,000 hours.
HCFL and CCFL employ entirely different mechanisms to generate electrons. The HCFL operates in the arc discharge region whereas the CCFL functions in the normal glow region. This is illustrated on page 339 from the book Flat Panel Displays and CRTS, edited by Lawrence E. Tannas, Jr., Von Nostrand Reinhold, New York, 1985, which is incorporated herein by reference. The HCFL functions in the arc discharge region. As shown in FIG. 10-5 on page 339 of this book, for the HCFL functioning in the arc discharge region, the current flow is of the order of 0.1 to 1 ampere. The CCFL functions in the normal glow region. Functioning in the normal glow region of the gas discharge, the current flow in the CCFL is of the order of 10xe2x88x923 ampere, according to FIG. 10-5 on page 339 of the above-referenced book. Thus, the current flow in the HCFL is about two orders of magnitude or more than that in the CCFL.
The HCFL typically employs a tungsten coil coated with an electron emission layer. For more details, see page 61 of Applied Illumination Engineering, Second Edition, Jack L. Lindsey, 1997, published by The Fairmont Press, Inc. in Lilburn, Ga. 30247, which is incorporated herein by reference. A xc2xd watt or 1 watt of power is needed to heat the tungsten coil to about 1,000xc2x0 C. At this temperature, the electrons can easily leave the electron emission layer and a small voltage of the order of about 10 volts will pull large currents into the discharge. The large current flow is in the form of a visible arc, so that the HCFL is also known as the arc lamp. The small voltage will also pull ions from the discharge which return to the tungsten coil, thereby ejecting secondary electrons. However, since the cathode-fall voltage (xcx9c10 V) is small, the sputtering effect of such ions would be small. The lifetime of an HCFL is determined primarily by the evaporation of the electron emission layer at the high operating temperature of the HCFL.
The CCFL emit electrons by a mechanism that is entirely different from that of the HCFL. Instead of employing an electron emission layer and heating the cathode to a high temperature to make it easy for electrons to leave the cathode, the CCFL relies on a high cathode-fall voltage (xcx9c150 V) to pull ions from the discharge. These ions eject secondary electrons from the cathode and the cathode- fall then accelerates the secondary electrons back into the discharge producing several electron-ion pairs. Ions from these pairs return to the cathode. Because of the high cathode-fall voltage (xcx9c150 V), the ions are accelerated by the cathode-fall voltage from the discharge to the cathode, thereby causing sputtering. Different from the HCFL, no power is wasted to heat the CCFL to a high temperature.
The HCFL operates at a relatively low voltage (xcx9c100 V) whereas the CCFL operates at high voltages (of the order of several hundred volts). The HCFL operates at a temperature of about 40xc2x0 C. and above, with the cathode operating at a relatively high temperature of about 1,000xc2x0 C., whereas the CCFL operates in a temperature range of about 30-75xc2x0 C., with the cathode operating at a temperature of about 150-190xc2x0 C. For further information concerning the differences between HCFL and a CCFL, please see the paper entitled xe2x80x9cEfficiency Limits for Fluorescent Lamps and Application to LCD Backlighting,xe2x80x9d by R. Y. Pai, Journal of the SID, May 5, 1997, pp. 371-374, which is incorporated herein by reference.
CCFLs typically comprise an elongated tube and a pair of electrodes where the current between the electrodes in the CCFL is not more than about 5 milliamps and the power delivered by the CCFLs less than about 5 watts. In order to increase the power delivered by the CCFL, it is possible to increase either the length of (and consequently, the voltage across the CCFL) or the current in the CCFL. It may be difficult to manufacture CCFLs whose tubes are excessively long. Furthermore, when the tube length of the CCFL is excessive, they must be operated at high voltage so that this increases the cost and reduces the reliability of the CCFL drivers. Another way to increase the power output of the CCFL is to increase the current in the CCFL. However, as noted above, because of the high cathode-fall voltage which may be about 150 V, ions are accelerated from the discharge towards the cathode, thereby causing sputtering. This means that if a large current is flowing in the CCFL, the return of the ions to the cathode may cause excessive sputtering, which drastically reduces the useful life of the CCFL.
None of the above-described gas discharge devices are entirely satisfactory. It is, therefore, desirable to provide an improved gas discharge device where the above-described disadvantages are not present.
This invention is based on the observation that the above-described sputtering caused by the return of the ions to the cold cathode may be reduced by distributing or spreading the current over two or more sub-electrodes rather than a single electrode, so that each sub-electrode is not required to carry excessive current. In this manner, the sputtering that does occur will not be excessive and will not drastically reduce the useful life of a cold cathode gas discharge system. This enables the cold cathode gas discharge system to be capable of being operated at higher current, while at the same time, the useful life of the system will not be significantly reduced by the larger current flow. This enables the system to provide higher power without significantly compromising the useful life of the system.